Capacitive touch panel technology is in wide use, for example in mobile phones, satellite navigation systems, PDA screens and handheld games consoles.
One particular form of capacitive touch panel is known as projective capacitive touch technology or “PCT”. In PCT devices, an XY array of sensing electrodes is formed in layers of transparent conducting material. In use, capacitance forms between the user's fingers and the projected capacitance from the sensing electrodes. A touch is made, precisely measured and translated into a command which is executed by underlying electronic devices for an appropriate software application. PCT screens enjoy the benefits of responding accurately to both fingers and styli.
One particular form of PCT technology has two separated layers of transparent conducting material and it is the changes in the mutual capacitance between the electrodes at the intersection points of the electrode array layer that are detected.
The layers of transparent conductive material are each divided into a plurality of discrete electrode cells which are electrically connected in a first orthogonal direction but electrically isolated in a second orthogonal direction. The scribed pattern may be the same for both layers or may be different.
Conventionally, in a capacitive touch panel using PCT technology, a transparent substrate has deposited on each of its opposite surfaces a layer of transparent electrically conducting material. An electrode pattern is scribed in each of the layers of transparent electrically conducting material using two separate lasers. Each laser scribes a pattern by focusing on the surface of the substrate closest to it, so-called near-side ablation. Patent application GB 08003305.3 describes such a method. Alternatively a single laser is used and the substrate may need to be turned over and the device aligned to carry out the scribing process for the second layer. Where the electrode pattern is desired to be identical on the two surfaces, a single laser can be focused between the two layers and its parameters selected to provide scribing in each of the two layers simultaneously without damage to the transparent substrate. It will be appreciated this one laser process cannot be used to provide different patterns on the two layers. This process is described in JP 8320638A and US 2008/129318 A.
WO 2011/018595 describes a process for providing scribing of different electrode patterns on both of the electrically conductive layers of a PCT touch panel, without the need for a second laser or to turn the substrate. This is achieved by changing the focal length of the laser optics from a point at the surface of the substrate closest to the laser to a point at the surface of the substrate farthest from the laser, to perform so-called far-side ablation. Conventionally, near-side ablation is the preferred mechanism by which to scribe an electrode pattern in an electrically conductive layer of a PCT touch panel. This is because laser power is attenuated by the glass substrate. The method described in WO 2011/018595 is an exception to this convention, which accepts laser attenuation in order to simplify the process by removing the need for a second laser or to turn the substrate and then recalibrate the laser.